Method and apparatus for allocating channelization codes for wireless communications

ABSTRACT

A data transmission method for a MIMO (Multiple-input Multiple-Output) system allowing multiple terminals to share channelization codes according to the channel states, by receiving channel information from terminals, allocating channelization codes for each transmit antenna based upon the received channel information, and transmitting data via each transmit antenna according to the allocated channelization codes. When there are many user terminals and if the number of channelization codes are limited, namely, when the total number of channel codes used to distinguish the terminals is less than the total number of terminals, channelization code sharing per transmit antenna among multiple terminals is performed, such that the throughput of data that can be sent by using each channelization code is increased.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to communication systems, more particularly, tomethods and apparatus for allocating channelization codes in a wirelesscommunication system with multiple antennas.

2. Description of the Background Art

Due to the rapid development of wireless communications, there is anincreasing need to provide various types of multimedia services (e.g.,video, photos, animation, music, games, etc.) through wirelessinterfaces supporting higher capacity data transmissions and higher datatransmission rates. Accordingly, methods for allowing more efficient useof limited bandwidth are becoming more critical. To address theseissues, new data transmission techniques using multiple antennas arebeing developed, and the so-called “MIMO” (Multiple-inputMultiple-Output) technique is one such example.

FIGS. 1 and 2 show the structures of a transmit end and a receive endfor a PARC (Per Antenna Rate Control) MIMO system.

As shown in FIG. 1, the transmit end of a MIMO system comprises ademultiplexer (Demux) 10 that separates (branches) an inputted highspeed data stream to multiple transmit antennas (Ant 1˜Ant n), a symbolcoding unit 11 that generates symbols by coding each sub-streamseparated from the inputted data stream, a channelization coding unit 12that allocates channelization codes (C1-Cn) to the symbols outputtedfrom the symbol coding unit 11, a adder 13 that adds the symbols codedby the channelization coding unit 12, a scrambling unit 14 thatallocates scrambling codes (S1˜Sn) to the symbols outputted from theadder 13, and multiple transmit antennas (Tx1˜Txn) that transmit thesymbols that have been spread by the scrambling unit 14. Note that thescrambling code S1˜Sn are usually same code.

The symbol coding unit 11 performs channel coding, interleaving, andmapping operations to generate symbols, and also performs the functionof separating the generated symbols. The channelization coding unit 12allocates channelization codes (C1˜Cn) to the separated symbols andspread the symbols, and the scrambling unit 14 scrambles the symbolsspread by channelization codes allocated thereto.

In the related art, the receive end of the MIMO system as shown in FIG.2 comprises an interference signal removal unit 20 that removesinterference signals from a received signal, a MMSE detector 21 thatperforms MMSE (Minimum Mean-squared Error) linear conversion uponfinding the signal having the largest SINR (Signal-to-Interference NoiseRatio) among those signals outputted from the interference signalremoval unit 20, a de-spreading unit 22 that de-spreads the output ofthe MMSE detector 21, a multiplexer (MUX) 23 that multiplexes thede-spread signals, a symbol detecting unit 24 that processes themultiplexed signal and detects a sub-stream therefrom, a signalreconstruct unit 25 that reconstructs the sub-stream detected by thesymbol detecting unit 24 into a format that is equivalent (or similar)to the received signal, and a signal collecting unit 26 that forms adata stream by collecting the sub-streams that were sequentiallydetected by the symbol detecting unit 24.

The interference signal canceling unit 20 comprises a plurality ofbuffers used for deleting the sub-streams reconstructed at the symboldetecting unit 24 from the received signal. The de-spreading unit 22comprises a plurality of de-spreaders to perform de-spreading by usingthe codes allocated to a corresponding sub-stream at the transmit end.

The related art MIMO system having the above structure performs thefollowing operations.

The high speed data stream is demultiplexed by the demultiplexer 10 intoa number of sub-streams equaling the total number of transmit antennas.Each demultiplexed sub bit stream undergoes channelization coding andinterleaving at the symbol coding unit 11 and then mapped to symbols.The corresponding symbols are again separated/branched (demultiplexed)according to the total number of channelization codes.

The channelization coding unit 12 allocates channelization codes (C1-Cn)to the branched symbols, and the symbols having channelization codesallocated thereto are added into one symbols at the adding unit 13. Thescrambling unit 14 allocates scrambling codes to the symbols outputtedfrom the adding unit 13 and then transmits via the transmit antennas(Tx1-Txn). Generally, the scrambling codes allocated to the symbols aresame code.

The MMSE detector 21 of the receiving end detects a signal having thelargest SINR (Signal-to-Interference Noise Ratio) among the signalsreceived via the receive antennas (Rx1-Rxn) and performs MMSE (MinimumMean-squared Error) linear conversion. The output of the MMSE detector21 is de-spread at the de-spreading unit 22 and is combined into onesignal at the multiplexer 23. The symbol detecting unit 24 detects thetransmit symbols from the signals output by the multiplexer 23, and thenperforms reverse mapping and reverse interleaving operations fordetection as a first stream.

The signal reconstruct unit 25 reconstructs the first sub-streamdetected by the symbol detecting unit 24 into the form of the receivesignal and then outputs to the interference signal canceling unit 20,which removes the reconstructed signal from the receive signal stored inthe buffer and then outputs to the MMSE detector 21.

Accordingly, the MMSE detector 21 performs MMSE (Minimum Mean-squaredError) liner conversion on the signal having the largest SINR(Signal-to-Interference Ratio) among the remaining signals, and theoutput of the MMSE detector 21 goes through the de-spreading unit 22 andthe multiplexer 23, and inputted to the symbol detecting unit 24, whichdetects a second sub-stream.

The signal reconstruct unit 25 reconfigures the second sub-streamdetected by the stream detecting unit 24 and outputs to the interferencesignal canceling unit 20, which removes the reconstructed signal fromthe signal stored in the buffer and outputs to the MMSE detector 20.Then, the above explained procedures are repeatedly performed such thatthe symbol detecting unit 24 sequentially detects sub-streams.

Thus, when all sub-streams are detected by the symbol detecting unit 24,the detected multiple sub-streams are collected by the signal collectingunit 26 to for a single data stream.

Hereafter, the method of allocating channelization codes to the transmitsymbols at the transmit end will be explained in more detail as follows.

FIG. 3 is a flow chart showing a channelization allocation method in amobile communications system according to the related art.

In general, when transmitted certain data to multiple terminals (e.g.,user equipment) (UE1˜UE8), a pilot signal is first transmitted forinitial synchronization with each terminal. The terminals (UE1˜UE8) thatreceive the pilot signal refer to the state of each transmit antenna andof each receive link, and transmit channel quality information (CQI) tothe base station. Here, the CQI can be, for example, a ‘transmitpossible bit number’, i.e., the number of bits (bps/Hz) that may betransmitted by each terminal for each transmit antenna.

The base station receiving CQI information from the terminals accordingto the transmit antennas, selects from the CQI information, a MCS(Modulation Code Set) that can be sent to each terminal in order tosupport a high data transmission rate (S10, S11). Then, the base stationselects a terminal having a high ‘transmit possible bit number’ withrespect to all the antennas, and allocates channelization codes to theselected terminal (S12). Namely, channelization codes are sequentiallyallocated to the symbols to be transmitted to the selected terminal(S13).

FIG. 4 shows an example of allocating channelization codes when thereare 4 transmit antennas, 8 users, and 4 channelization codes.

As shown in FIG. 4, because it is assumed that the maximum ‘transmitpossible bit number’ is largest for UE1 and smallest for UE 2, namely,the order according to ‘transmit possible bit number’ size isUE1>UE3>UE8>UE7>UE6>UE4(=UE5)>UE2, the base station sequentiallyallocates C1, C2, C3, and C4 respectively to each of the symbols of UE1,UE3, UE5, and UE6 to be transmitted via the four transmit antennas(Tx1˜Tx4), while channelization codes are not allocated to the remainingterminals (UE2, UE4, UE5, and UE6).

When allocation of channelization codes is completed, the base stationmultiplexes the symbols of the terminals having channelization codesallocated thereto and transmits through each transmit antenna (Tx1˜Tx4)(S14, S15).

In general, for a MIMO system using multiple antennas at the transmitand receive ends, the channel quality for each transmit antenna isdifferent. Thus, even if the link quality between the base station andthe terminals is considered to be good, in actuality, certain links maybe considered extremely good while other links may be consideredextremely bad. In such situations, link adaptation procedures, such asAMC (Adaptive Modulation and Coding) are used to apply higher modulationmethods (such as QAM (Quadrature Amplitude Modulation)) in order tomaximize system throughput.

When many users simultaneously receive services in a MIMO system, thebase station uses channelization codes to distinguish each user.However, if the total number of channelization codes is less than thetotal number of users, and the related art channelization codeallocation method is employed, there are situations where limitedchannel resources cannot be efficiently used. Namely, as shown in FIG.4, when there are eight users and four channelization codes are used,certain terminals (i.e., UE2, UE4, UE5, and UE6) are not allocated withany channelization codes, and thus the efficiency of using channelresources is undesirably decreased. This is because the related artchannelization code allocation method performs allocation ofchannelization codes by determining the maximum ‘transmit possible bitnumber’ (i.e., the channel state or condition) for each antenna.

Additionally, if the quality of the links between the base station andthe terminals are all considered to be bad (namely, when some linksbetween the transmit/receive antennas are good, while other links arebad), a low MCS (Modulation and Coding Set) is used or data streams arenot even transmitted for those channels allocated to the bad linksaccording to the related art. Thus, efficiency of channel usage isdegraded, and in particular, the data stream detection performance atthe receive end is degraded.

The present invention also relates to transmitting signals in a MIMO(Multiple-Input Multiple-Output) radio communication system to allowmore efficient use of resources.

Transmission methods employing antenna signal processing techniques canbe classified into open-loop procedures and closed-loop procedures. Theopen-loop procedure is a method that can implement code re-usingtechniques together with antenna diversity techniques, whereby a mobilestation need not provide feedback of channel information to the basestation. The closed-loop procedure employs weight information (W), usedfor the multiple antennas, obtained by using channel information that ismeasured at and fed back from the receiving end, and correspondingweight values are applied to each antenna for signals to be transmitted.

The MSB (Multi-Stream Beam-forming) or PSRC (Per-Stream Rate Control)MIMO system is one kind of FDD (Frequency Division Duplex) typeclosed-loop MIMO system.

FIG. 7 depicts a MSB MIMO system configuration according to the relatedart. The transmitting end 710 comprises a demultiplexing unit 711 thatreceives source data bits, a modulation unit 713 that performsmodulation allocation to each symbol sent from the demultiplexing unit711 and that receives feedback from the receiving end (explainedhereafter), a weight vector multiplying unit 715 that multiplies aweight vector to each modulation allocated symbol sent from themodulation unit 713 and that receives feedback from the receiving end(explained hereafter), and a plurality of transmit antennas (Tx 1through Tx M) that allows signal transmission through a MIMO channel.

The transmitting end considers the magnitude of each eigenvalue throughthe Eigen-decomposition of the channel matrix that is estimated by thereceiving end. For those channel regions having relatively largeeigenvalues, high order modulation methods (such as 64 QAM, 16 QAM, andthe like) are applied, while for those channel regions having relativelysmall eigenvalues, low order modulation methods (such as, BPSK, QPSK,and the like) are applied. In order to maintain the independence of thesymbols that are transmitted through respectively different datastreams, eigenvectors of the channel matrix are respectively multipliedto the symbols, which are then transmitted.

Namely, the same modulation method is not used for all the symbolstransmitted by each transmission stream, but the channel conditions(i.e., state or status) of the transmission end are received as feedbackfrom the terminals, and the appropriate modulation method for eachstream is used according to the channel conditions. Here, it can beunderstood that the receiving end requires the same algorithm as thatused in the transmitting end in order to determine the modulation methodused for each antenna for reception.

Also, for the symbols transmitted in a respectively different manner viaeach stream, the eigenvectors obtained by Eigen-decomposition of thechannel matrix are respectively multiplied to the symbols and thentransmitted, which allows channel region correlation to be utilized tothe maximum, and independence among transmission streams is maintained.

For relatively small channel regions with eigenvalues (obtained byEigen-decomposition of the channel matrix) of less than a particularvalue, symbols are not even transmitted in order to prevent thepossibility of errors being generated at the receiving end for thesymbols transmitted from the transmitting end.

Even so, because the symbols are allocated such that higher ordermodulation is used for streams having good channel conditions, the totalnumber of bits to be transmitted via all the transmit antennas isincreased, thus aiding the increase of throughput. Accordingly, theoverall communication quality can be improved by having the transmittingend prevent (in advance) the transmission of symbols in the channelregion created by using eigenvectors with small eigenvalues, wherebysymbol transmission would be useless anyway.

As shown in FIG. 7, the MSB MIMO system has an M number of transmitantennas and an N number of receive antennas. If H refers to the channelmatrix that the signal vectors (respectively transmitted differently viathe M transmit antennas) go through prior to being received at thereceiving end, the receiving end estimates the channel matrix (H), andthen sends to the transmitting end by feedback, each eigenvalue andeigenvector value obtained by Eigen-decomposition performed at thereceiving end. Alternatively, the eigenvalues can be compared and if thetransmitting and receiving ends have a pre-defined modulation methodallocation table, an index value (also stored in the table)corresponding to the modulation method to be used for each stream may befed back to the transmitting end.

As shown in FIG. 7, the receiving end 720 comprises a plurality ofreceive antennas (Rx 1 through Rx N) that receives signals from thetransmitting end 710 via a MIMO channel, a detecting unit 721 thatprocesses the received signal by using zero-forcing or MMSE methods, aconjugate multiplying unit 723 that multiplies conjugate values (of theweight vectors multiplied at the transmitting end 710) to the symbolsoutputted from the detecting unit 721, a demodulation unit 725 thatperforms demodulation by using the modulation allocated to the symbols,and a multiplexing unit 727 that performs multiplexing to outputreception data. Furthermore, there is a channel estimator 722 that alsoreceives the signals from the receive antennas (Rx 1 through Rx N) andan Eigen-decomposition unit 724 that processes the output of the channelestimator 722, and the results are fed back to the transmitting end 710.Here, the eigenvector information can be fed back to the weight vectormultiplying unit 715, while the eigenvalue information can be fed backto the modulation unit 713.

The receiving end estimates (deduces) the signal vectors that aregenerated at the transmitting end by performing beam-forming on eachsymbol, by employing the generally well-known zero-forcing techniques orMMSE techniques. Then, conjugate values of the weight vectors (that weremultiplied to each symbol at the transmitting end) are multiplied to thesignal vectors to obtain (detect) each symbol. Thereafter, the symbolstransmitted via each transmit antenna are modulated according to thecorresponding modulation method, to thus determine the bits that weretransmitted from the transmitting end, and multiplexing is performed tore-configure the bit streams transmitted from the transmitting end.

For the MSB MIMO system of the related art, a method for forming a beamfor each symbol can be summarized by the following Equation (1):S=w ₁ s ₁ + . . . w _(M) s _(M)  (1)

Here, w_(i) refers to a weight vector used in beam-forming for eachsymbol, s₁ through s_(M) refer to data symbols, and S refers to a signalvector after beam-forming is performed for each symbol. Each of thesymbols s₁, . . . , s_(M) is constructed from independent bit streamsoutputted from an M number of independent coding blocks.

The symbols transmitted from each transmit antenna are not modulatedusing the same modulation method, but the channel condition (state)information of each antenna are received as feedback from the terminals,and modulation is performed by to the modulation method determinedaccording to each channel condition, respectively.

Thus, for the symbols s₁ through s_(M) in Equation (1), the eigenvaluesof the channel matrix are compared and respectively differentmodulations are employed. Namely, upon performing Eigen-decomposition ofthe channel matrix, a higher order modulation is allocated to thosestreams having a relatively high eigenvalue, and a lower ordermodulation is allocated to those streams having a relatively loweigenvalue. Here, it is assumed that an eigenvector corresponding to thelargest eigenvalue is multiplied to the stream having a subscript of 1,and for the other streams thereafter, eigenvectors are multipliedsequentially thereto according to the order of magnitude of theeigenvalues.

The method for obtaining the weight vectors to be multiplied to eachsymbol can be summarized as follows.

If H refers to the channel matrix that the signal vectors (respectivelytransmitted differently via the M transmit antennas) go through prior tobeing received at the receiving end, and if the receiving end has Nreceive antennas, the channel matrix is a N*M matrix. If a pilot symbol(that is already known by the transmitting and receiving ends) or aseparate pilot channel is transmitted from each antenna of thetransmitting end, the receiving end can deduce each component of thechannel matrix.

For the channel matrix H, the receiving end performsEigen-decomposition. In this system, because it is assumed that thenumber of antennas in the antenna array at the transmitting end isgreater than that at the receiving end, the channel matrix H is not asquare matrix, and thus Eigen-decomposition cannot be performed on thechannel matrix itself, thus Eigen-decomposition of H ^(H) H is performedinstead.

Here, H refers to a Hermitian calculation, λ_(M) refers to theeigenvalues of the matrix H ^(H) H, and e_(M) refers to eigenvectors.

In general, because each eigenvector maintains orthogonalcharacteristics, signals can be transmitted by multiplying independentweight vectors to each symbol, when symbols are to be transmittedaltogether from the transmitting end according to the number of antennasat the transmitting end.

As in Equation (1), if a signal is to be transmitted upon performingbeam-forming on each symbol, the receiving end performs the followingsignal processing. As the signal was transmitted after multiplyingindependent weight vectors to each symbol, S can be deduced by using thegenerally known zero-forcing or MMSE methods, and then weight vectorsare multiplied to each symbol at the transmitting end for transmittingthe signal from the transmitting end, whereby the signal received by thereceiving end can be expressed as the following Equation (3):R=HS+n  (3)whereby, n refers to AWGN (Additive White Gaussian Noise).

If the signal vector S transmitted upon multiplying a weight vector toeach symbol by using zero-forcing is deduced as being the signal vectorŜ, this signal vector can be expressed as the following Equation (4):Ŝ=[H ^(H) H] ⁻¹ H ^(H) R  (4)

If the signal vector S transmitted upon multiplying a weight vector toeach symbol by using MMSE is deduced as being the signal vector Ŝ, thissignal vector can be expressed as the following Equation (5):Ŝ=[αI+H ^(H) H] ⁻¹ H ^(H) R  (5)

Here, a refers to the signal to interference noise ratio, and I refersto an identity matrix. If the signal vector S transmitted uponmultiplying a weight vector to each symbol by using zero-forcing or MMSEmethods is deduced as being the signal vector Ŝ, and the conjugatevalues of the weight vectors (that were multiplied to each symbol andtransmitted from the transmitting end) are multiplied to the deducedsignal vector so that the symbols s₁ through S_(M) transmitted from thetransmitting end can be deduced to obtain the deduced symbols ŝ₁ throughŝ_(M), which can be expressed as the following Equations (6):ŝ₁=w₁ ^(H) Ŝ . . . ŝ_(M=w) _(M) ^(H) Ŝ  (6)

The deduced symbols s, through SM are then modulated according to theirrespective modulation methods so that the bits prior to allocation toeach symbol can be determined. Also, after the bits of the symbolstransmitted via each antenna, the bit stream transmitted from thetransmitting end is determined through multiplexing.

When respectively different symbols are transmitted from each transmitantenna, in addition to transmission upon multiplying eigenvectors (ofthe channel matrix) to each symbol, the receiving end also relativelycompares the eigenvalues (of the channel matrix) obtained throughEigen-decomposition in order to consider the channel state (condition)of each antenna for determining the channel coding and modulation methodfor the symbols to be transmitted via each antenna.

To obtain the weight vector that is to be multiplied to the symbols tobe transmitted independently from each transmit antenna,Eigen-decomposition of the deduced channel matrix must be performed atthe receiving end. Here, no additional calculations are needed, becauseeigenvalues and eigenvectors can be obtained together throughEigen-decomposition.

Namely, the same modulation method is not applied to the symbolstransmitted from each transmit antenna, but each channel region state isreceived as feedback from the terminals, and the appropriate modulationmethod to be used is determined accordingly for each stream based uponthe channel state. Thus, by comparing the eigenvalues of the channelmatrix for s₁ through s_(M) in Equation (1), respectively differentmodulation methods are employed.

Thus, upon performing Eigen-decomposition of the channel matrix, ahigher order modulation is allocated to those streams having arelatively high eigenvalue, and a lower order modulation is allocated tothose streams having a relatively low eigenvalue.

Of course, after determining the channel coding method and modulationmethod of the symbols to be transmitted in each stream, there are nochanges in performing independent beam-forming of each symbol by usingthe eigenvectors of the channel matrix obtained at the receiving end asweight vectors. Here, the method of determining the modulation andcoding scheme (MCS) to be allocated to each antenna is as follows.

To determine the modulation method to be allocated to each stream usingthe eigenvalues obtained by the Eigen-decomposition as shown in Equation(2), the respective ratios (λ₁:λ₂: . . . :λ_(M)) of each eigenvalue isfirst determined.

Through these ratios, if the smallest eigenvalue is less than athreshold value, symbols are not transmitted in the channel regioncreated by the corresponding eigenvector. For the next smallesteigenvalue, a lower order MCS (modulation and coding set) is used, whilea higher order MCS is used for the largest eigenvalue. For intermediateeigenvalues, an intermediate order MCS is used upon relative comparisonof such values.

Then, after confirmation by testing different channel states(conditions) through simulations, a modulation allocation table may beused to select the desired modulation and coding combination that isappropriate for the channel state.

However, when multiple terminals receive a service in the related artMSB MIMO system, channelization codes are used to distinguish theterminals. Multiple data streams transmitted simultaneously by oneterminal can be distinguished by the orthogonal characteristics ofeigenvectors.

FIG. 8 depicts a table to explain the method of allocating radioresources in a related art MIMO system when multiple terminalssimultaneously receive service, and the number of channelization codesare less than the number of terminals. It shows an example where thereare 8 terminals, and 4 channelization codes are used. Here, onlyterminals 1, 3, 7, and 8 are allocated with channelization codes and cancommunicate, while terminals 2, 4, 5, and 6 are not allocated withchannelization codes (even though there are available radio resources)and thus cannot communicate.

As shown in FIG. 8, if the number of channelization codes are less thanthe number of terminals in the related art MIMO system, channelresources cannot be allocated due to the lack of channelization codes,even though there are available channel resources.

Also, excluding the situation when the quality of all the channel linksbetween the base station and multiple terminals are good, namely, whensome transmit channel links are good while others are bad, the datastreams allocated to the bad links are not even transmitted or aretransmitted by using lower order MCS (modulation and coding schemes),thus channel resources are not efficiently utilized.

SUMMARY OF THE INVENTION

Therefore, a first object of the present invention is to provide anapparatus and method for allocating channelization codes in a mobilecommunication system capable of enhancing a symbol detection capabilityof a receiving end.

A second object of the present invention is to provide an apparatus andmethod for allocating channelization codes in a mobile communicationsystem capable of improving usage efficiency of a limited channelresource.

A third object of the present invention is to provide an apparatus andmethod for allocating channelization codes in a mobile communicationsystem capable of allowing users to share channelization codes byantennas.

A fourth object of the present invention is to provide an apparatus andmethod for allocating channelization codes in a mobile communicationsystem capable of increasing a throughput of data that can be sentthrough channelization codes.

To achieve at least the above objects in whole or in parts, there isprovided a transmission method for a multiple antenna communicationsystem, the method comprises receiving channel information fromterminals, allocating channelization codes for each transmit antennabased upon the received channel information, and transmitting data viaeach transmit antenna according to the allocated channelization codes.

To achieve at least the above objects in whole or in parts, there isprovided a base station apparatus having multiple transmit antennas, theapparatus comprises a receiver that receives channel information fromterminals, an allocating unit that allocates channelization codes foreach transmit antenna based upon the received channel information, and atransmitter that transmits data via each transmit antenna according tothe allocated channelization codes.

The inventors have recognized and addressed at least the above-explainedshortcomings of the related art by developing a technique for improvedsymbol detection performance at the receive end of a MIMO system whenallocating channelization codes in a mobile (wireless) communicationsystem. As a result, the efficiency of using channel resources can beimproved, data throughput can be increased, and channelization codes canbe shared between users according to the transmit antennas.

In a broad sense, the aspects of the invention relate to allocation ofchannelization codes for each antenna according to the channel qualitybetween the transmit and receive antennas. The channel quality may bedetermined based upon channel quality information (CQI) sent from eachterminal, and the CQI can be mapped by using the ‘transmit possible bitnumber’ of each antenna for each terminal.

The channel quality information is received, channelization codes aresequentially allocated to each antenna according to the channel qualityinformation, and data transmission is then performed.

Also, the inventors recognized and addressed the problems of the relatedart by developing a signal transmitting method for a MIMO system inwhich channelization codes are shared by multiple terminals when thenumber of terminals is greater than the number of channelization codes,such that radio resources are more efficiently utilized.

BREIF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a transmit end of a PARC (Per Antenna Rate Control) MIMOsystem according to the related art.

FIG. 2 depicts a receive end of a PARC (Per Antenna Rate Control) MIMOsystem according to the related art.

FIG. 3 depicts a flow chart showing a channelization code allocationmethod for a mobile communication system according to the related art.

FIG. 4 depicts an example for FIG. 3 of allocating channelization codeswhen there are four transmit antennas, eight users, and fourchannelization codes.

FIG. 5 depicts a flow chart showing a channelization code allocationmethod for a mobile communication system according to an embodiment ofthe present invention.

FIG. 6 depicts an example for FIG. 5 of allocating channelization codeswhen there are four transmit antennas, eight users, and fourchannelization codes.

FIG. 7 depicts a blocks diagram of a MIMO system according to therelated art.

FIG. 8 depicts a table used to explain an allocation method for radioresources when the number of channelization codes is less than thenumber of terminals and multiple terminals simultaneously receive aservice according to the MIMO system of the related art.

FIG. 9 depicts a block diagram of a MIMO system according to anembodiment of the present invention.

FIG. 10 depicts a table used to explain an allocation method for radioresources when the number of channelization codes is less than thenumber of terminals and multiple terminals simultaneously receive aservice in the MIMO system according to an embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In general, the channel quality of each transmit antenna in a MIMOsystem employing multiple antennas at the transmit and receive ends.Namely, even if the overall channel quality of the transmit antennaswith respect to a particular terminal (user) is satisfactory, thechannel quality between each antenna and various terminals may not besatisfactory. Thus, even if the link quality between the base stationand the terminal is considered to be good, in actuality, there may besome links that are considered very good, while other links that areconsidered not good.

Accordingly, unlike the related art that considers the maximum ‘transmitpossible bit number’ with respect to all antennas when allocatingchannelization codes, the invention provides a scheme that determinesthe maximum ‘transmit possible bit number’ (i.e., the channel state orcondition) for each antenna when allocating channelization codes.

Also, when multiple users simultaneously receive a service in a MIMOsystem according to the related art, if the number of channelizationcodes is less than the number of users, there are situations where auser with satisfactory channel quality for a particular transmit antennais not allocated with channelization codes, although the channel qualityof the overall transmit antennas are not satisfactory.

Thus, the invention provides a scheme in which channelization codes canbe shared between users according to the transmit antennas, to allowmore efficient use of the limited number of channelization codes.

FIG. 5 depicts a flow chart showing a channelization code allocationmethod for a mobile communication system according to an embodiment ofthe present invention.

As shown in FIG. 5, when data (i.e., a data stream) is to be transmittedto multiple terminals (UE1˜UE8), the base station first transmits apilot signal for initial synchronization with the terminals.

The terminals (UE1˜UE8) receiving the pilot signal determine the state(condition) of each link between the transmit antennas (array) and thereceive end, and transmits to the base station CQI data indicating thelink quality. Here, the CQI data can be mapped by, for example, transmitpossible bits (bps/Hz) according to each transmit antenna.

The base station receives CQI data for each antenna from the terminals(S20), and selects from the corresponding CQI, the MCS (Modulation CodeSet) that can be sent to each terminal (S21). Then, the base stationselects one antenna and then selects a plurality of terminals that havea large transmit possible bit number, whereby the total number ofselected terminals equal the number of channelization codes (S22, S23).After the terminals are selected, the base station sequentiallyallocates channelization codes to the selected terminals (S24).

FIG. 6 depicts an example of allocating channelization codes when thereare four transmit antennas, eight users, and four channelization codes.

As shown in FIG. 6, the ‘transmit possible bit number’, i.e., the numberof bits that can be transmitted to the terminals UE1, UE6, UE7, and UE8via the transmit antenna (Tx1) is 4 bps/Hz, the number of bits that canbe transmitted to terminal UE3 is 2 bps/Hz, and the number of bits thatcan be transmitted to terminal UE5 is 0 bps/Hz.

Accordingly, if the transmit antenna Tx1 is first selected, the basestation will select the terminals that have a large ‘transmit possiblebit number’ (i.e., UE1, UE6, UE7, and UE8 are selected), andchannelization codes (C1˜C4) will be sequentially allocated to thesymbols that are to be transmitted to the selected terminals (UE1, UE6,UE7, and UE8). Due to this, when a plurality of channelization codes areused in transmitting symbols to multiple terminals (users), theterminals can share the channelization codes, and those terminals thatshare the same channelization code receive the symbols transmitted viarespectively different antennas.

Then, the base station checks whether code allocation for thecorresponding transmit antenna (Tx1) has been completed (S25),repeatedly performs the steps after S23 if code allocation is notcomplete, and checks to see if there are any other remaining antennas ifcode allocation is completed for the corresponding antenna (S26).

If remaining antennas exist as a result of the checking step, the stepsafter S22 are repeatedly performed with respect to transmit antennas Tx2through Tx4, and if no remaining antennas exist as a result of thechecking step, the base station multiplexes and transmits the symbols ofthe terminals that have been allocated the channelization codes througheach transmit antenna (Tx1˜Tx4) (S27).

Accordingly, as shown in FIG. 6, if the channelization code allocationmethod of the present invention is employed, a greater number ofterminals can use the limited number of channelization codes, becausemultiple terminals can share a channelization code according to eachtransmit antenna, unlike the related art method of using one particularchannelization code for one particular terminal. In particular, thepresent invention allows channelization codes to be shared by usersaccording to transmit antennas, to thus increase the throughput of thedata that can be transmitted by using each channelization code. Namely,channelization code C1 can be used to transmit a data stream of 48bps/Hz in the related art, but the present invention allows the samechannelization code C1 to be used for transmitting a data stream of 58bps/Hz.

According to the present invention, the data transmission method in aMIMO (Multiple-input Multiple-Output) system allows allocation of thesame channelization code to more than one terminal, if the total numberof channelization codes is less than the total number of terminals thatreceive a particular service.

When channelization codes are shared by multiple terminals, it isnecessary to distinguish among the terminals. To do so, channelizationcode sharing is permitted only if the relative related values of theweight vectors that are multiplied to the data streams of the terminalsare smaller than a particular threshold value. Thus, with respect toeach terminal, a certain level of interference (caused by the effects ofcode sharing) from other terminals is permitted so that data streamsamong multiple terminals can be distinguished.

The MSB (Multi-Stream Beam-forming) MIMO system of the present inventionassumes that there are M transmit antennas, each data stream thatemploys the same channelization code (such as in PARC MSB) areseparately coded, and CRC (cyclic redundancy check) codes are addedthereto, respectively.

FIG. 9 shows an exemplary structure of a closed-loop MSB MIMO systemaccording to the present invention. It can be understood that thestructure according to the present invention is similar to that of therelated art FIG. 7, but a control block (e.g., a multi-user weightcontroller 917) that considers multiple user terminals and provides theappropriate signal processing is additionally employed. Namely, themulti-user weight controller 917 receives feedback from theEigen-decomposition unit 924 that outputs both eigenvector feedback andeigenvalue feedback. The multi-user weight controller 917 processes thisfeedback information and provides outputs to the multiple demultiplexingunits (911 a˜911 c), the modulation unit 913, and the weight vectormultiplying unit 915, for performing the allocation method of thepresent invention described in more detail below.

FIG. 10 depicts a table used to explain an allocation method for radioresources when the number of channelization codes is less than thenumber of terminals and multiple terminals simultaneously receive aservice in the MIMO system according to an embodiment of the presentinvention.

As an example, there are 8 terminals (i.e., user equipment: UEs) and 4channelization codes, which are shared among the terminals. whereby onlyterminals 1, 3, 7, and 8 are allocated with channelization codes and cancommunicate, while terminals 2, 4, 5, and 6 are not allocated withchannelization codes (even though there are available radio resources)and thus cannot communicate.

Here, it can be understood that the data streams for the terminalshaving channelization codes C1, C2, C3, C4 can be clearly distinguished.Namely, the data streams 1, 2, and 3 of UE 1; the data stream 1 of UE 2;the data streams 1, 2, and 3 of UE 3; and the data streams 1 and 2 of UE6 all use respectively different channelization codes, thus can bedistinuighed without interference. However, for example, the datastreams 1, 2, and 3 of UE 1 and data stream 1 of UE 4 use to samechannelization code, thus a way of distinguishing these is required.

Due to the characteristics of the MSB (Multi-Stream Beam-forming)technique, the same data streams of a terminal, i.e., data streams 1, 2,and 3 can be distinguished by the eigenvector attributes havingrespectively orthogonal characteristics.

The eigenvector used for data stream 1 of UE 4 does not have orthogonalcharacteristics with the eigenvectors used for data streams 1, 2, and 3of UE 1. Thus, in order to share channelization codes among differentterminals (such as UE 1 and UE 4), the related value between theeigenvectors multiplied to the data symbols to be transmitted bychannelization coding, must be below a particular threshold value. Thiscan be expressed by the following Equation (7):S=W ₁ ¹ s ₁ ¹ +w ₂ ¹ s ₂ ¹ +w ₃ ¹ s ₃ ¹ +w ₁ ⁴ s ₁ ⁴  (7)Here, w_(i) ^(k) refers to the eigenvector multiplied to the i^(th)stream of the k^(th) terminal, s_(i) ^(k) refers to a symbol generatedfrom the independent bit stream outputted from the i^(th) independentcoding block of the k^(th) terminal, S refers to the signal vector afterbeam-forming is performed on each symbol. Namely, because there are noorthogonal characteristics between the weight vector set w₁ ¹, {w₂ ¹, w₃¹} of UE 1 and the weight vector set {w₁ ⁴} of UE 4, channelization codesharing is allowed only when the related value (r_(i,1) ^(1,4)=w_(i)¹·w₁ ⁴ (i=1,2,3)) among the elements of these two sets is below aparticular threshold value. When the above technique is expressed in amore general manner, the conditions for the weight value set of multipleterminals that allows channelization code sharing can be expressed asthe following Equation (8):{w _(i) ^(k) |w _(i) ^(k) ·w _(j) ^(l) <C _(th) , k=1, . . . , K, l=1, .. . , K, i=1, . . . , M _(k) , j=1, . . . , M _(l)}  (8)Here, k and i refer to the terminal number, subscripts i, j refer to theindex of the eigenvector for each terminal.

It is well known that employing multiple antennas at the transmittingend and the receiving end results in an increase of data transmissionrate and improvement of communication quality. For a MIMO systemproviding services to many terminals (users) and if the number ofchannelization codes are limited, the present invention provideschannelization code sharing per transmit antenna among multipleterminals, such that the throughput of data that can be sent by usingeach channelization code is increased.

Thus, the present invention provides a transmission method for amultiple antenna communication system, the method comprising the stepsof receiving channel information from terminals, allocatingchannelization codes for each transmit antenna based upon the receivedchannel information, and transmitting data via each transmit antennaaccording to the allocated channelization codes.

Also, as this method can be implemented in the transmitting end of acommunication system, the present invention provides a base stationapparatus having multiple transmit antennas, comprising, a receiver thatreceives channel information from terminals, an allocating unit thatallocates channelization codes for each transmit antenna based upon thereceived channel information, and a transmitter that transmits data viaeach transmit antenna according to the allocated channelization codes.

As described above, the present invention sequentially allocateschannelization codes to each transmit antenna according to the channelquality (i.e., the maximum number of bits that can be transmitted) foreach antenna by utilizing the fact that the channel quality for eachtransmit antenna is different. Namely, the base station allocateschannelization codes sequentially to users having a good quality channelstate with respect to each antenna based upon the CQI (channel qualityinformation) transmitted from the terminals. As a result of allocatingchannelization codes in this manner, a channelization code can be sharedamong multiple users, and each of these users can receive datatransmitted via different antennas.

Accordingly, the present invention can allow transmission of symbols byusing the most optimal channel, thus resulting in a more efficient useof channel resources compared to the related art in which onechannelization code was limited to use for only one user. In particular,the present invention allows the sharing of channelization codes amongmultiple users according to transmit antennas, resulting in a higherthroughput of data that can be transmitted trough the use of eachchannelization code.

Additionally, the present invention has been described with reference tothe included drawings only for exemplary purposes. Those skilled in theart would understand that various modifications and equivalent otherembodiments of the present invention are possible. Thus, the technicalspirit the following claims defines the scope of technical protectionfor the present invention.

1. A transmission method for a multiple antenna communication system,the method comprising: receiving channel information from terminals;allocating channelization codes for each transmit antenna based upon thereceived channel information; and transmitting data via each transmitantenna according to the allocated channelization codes.
 2. The methodof claim 1, wherein the channel information relates to the number ofbits that can be transmitted via each antenna for each terminal.
 3. Themethod of claim 1, wherein the channel information relates to a quality,a condition, or a state of the channel to be used for data transmission.4. The method of claim 1, wherein the allocating of channelization codesis performed sequentially to multiple terminals according to the numberof bits that can be transmitted.
 5. The method of claim 4, wherein thesequential allocation is performed by first allocating channelizationcodes to those terminals that can transmit the greatest number of bits,and then to those terminals that can transmit the next greatest numberof bits.
 6. The method of claim 4, wherein the allocating results in asingle channelization code being shared by multiple terminals.
 7. Themethod of claim 1, wherein the receiving, allocating, and transmittingsteps are performed in a closed-loop MIMO (Multiple-InputMultiple-Output) communication system.
 8. The method of claim 7, whereinthe MIMO system employs a MSB (Multi-Stream Beam-forming) technique. 9.The method of claim 8, wherein allocating the same channelization codeto multiple terminals based upon the channel states, when the number ofchannelization codes used to distinguish terminals is less than thenumber of terminals receiving a service.
 10. The method of claim 9,wherein the same channelization code is allocated only when a relatedvalue of a set of weight values for those terminals that share saidchannelization code, is below a particular threshold value.
 11. A basestation apparatus having multiple transmit antennas, the apparatuscomprising: a receiver that receives channel information from terminals;an allocating unit that allocates channelization codes for each transmitantenna based upon the received channel information; and a transmitterthat transmits data via each transmit antenna according to the allocatedchannelization codes.
 12. The apparatus of claim 11, wherein the channelinformation relates to the number of bits that can be transmitted viaeach antenna for each terminal.
 13. The apparatus of claim 11, whereinthe channel information relates to a quality, a condition, or a state ofthe channel to be used for data transmission.
 14. The apparatus of claim11, wherein the allocating of channelization codes is performedsequentially to multiple terminals according to the number of bits thatcan be transmitted.
 15. The apparatus of claim 14, wherein thesequential allocation is performed by first allocating channelizationcodes to those terminals that can transmit the greatest number of bits,and then to those terminals that can transmit the next greatest numberof bits.
 16. The apparatus of claim 14, wherein the allocating resultsin a single channelization code being shared by multiple terminals. 17.The apparatus of claim 11, wherein the receiving, allocating, andtransmitting steps are performed in a closed-loop MIMO (Multiple-InputMultiple-Output) communication system.
 18. The apparatus of claim 17,wherein the MIMO system employs a MSB (Multi-Stream Beam-forming) orPSRC (Per Antenna Rate Control) technique.
 19. The apparatus of claim18, wherein allocating the same channelization code to multipleterminals based upon the channel states, when the number ofchannelization codes used to distinguish terminals is less than thenumber of terminals receiving a service.
 20. The apparatus of claim 19,wherein the same channelization code is allocated only when a relatedvalue of a set of weight values for those terminals that share saidchannelization code, is below a particular threshold value.